Among fuels derived from plant biomass, ethanol has received particular attention as a potential replacement for or supplement to oil-derived products. Production of ethanol from biomass is normally obtained through the fermentation process of raw biological material rich in sugar or starch such as grain, sugarcane or corn also referred as first generation bio-ethanol.
To minimize the production cost and increase the potential of bio-ethanol produced from biomass, it is crucial to use lignocellulosic biomass in the form of low-cost byproducts from gardening, agriculture, forestry, timber industry and the like; thus for example, materials such as straw, maize stems, forestry waste, sawdust and wood-chips. Ethanol produced from this type of biomass is also referred to as second generation bio-ethanol.
Lignocellulosic biomass contains sugar polymers in form of hemicelluloses and cellulose. Before those sugars can be fermented to ethanol the sugar polymers has to be broken down to its sugar monomers. A common way to brake down the polymers is to use enzymatic hydrolysis. To increase the availability of the biomass to the enzymes the lignocellulosic biomass often undergoes a thermo/chemical pre-treatment.
Following a thermochemical approach such a process often requires that the temperature of the biological matter is raised to a temperature being above the boiling temperature of the liquid in which the biological matter is contained. Therefore, the skilled person is often faced with the problem of pressurising the slurry or pulp containing the biological matter so that the temperature can be increased to temperatures above the boiling temperature of the liquid while maintaining the fluid in a liquid state.
Such processes are often carried out in a container, such as a reactor chamber in which the pressure and temperature in many practical instances are elevated relatively to the atmospheric conditions while the raw biological material is stored and taken from a reservoir at atmospheric conditions. This is prone to trick safety risk and other issues relating thereto as the elevated pressure in the reactor will tend to force material upstream in the apparatus and in some cases a so-called back flash of very hot steam, ammonia, or in general chemicals and other gaseous mixtures out of the reactor and upstream in the bio-ethanol production system may occur. Such back flash may result in e.g. breakdown and destruction of the system and in some cases cause danger to humans operating the system. Thus there is a problem related to the prevention of back flash from occurring while allowing in-feed of biological material into the reactor.
In a further aspect, an efficient production system may require that the process in the reactor is carried continuously and in such aspects, there is a further problem relating to the in-feed needs to be continuous.
Suggestions as to solving this problem have been put forward. For instance, U.S. Pat. No. 4,270,976 suggests the use of a plug screw feeder to transport a pulp towards a reactor. The screw feed is constructed so that the pulp is dewatered and compacted during its transportation towards an inlet of the reactor with the aim of producing a “bio plug” that is intended to produce a seal of the reactor. This approach has turned out not to remedy the problem and requires a needy greedy control in combination of the water content, type, comminution, amount, particle size and distribution of the biomass introduced and in-feed velocity to obtain the plug. Naturally, the constitution of the raw material varies within broad limits and the end result is that safety issue has not been solved and the process is not very effective due to the produced plug does not always produce a tight seal.
In order to increase the availability of bio plug methods different hydraulic measures have been applied. For instance in WO0007806 such hydraulic measures includes a reciprocating movement of a screw feeder for providing a further compacting of the bio plug or includes a valve at the end of the screw feeder that can close the inlet to the reactor.
EP 1775376 discloses an apparatus for making wood fibres from wood chips by feeding the chips through a tamping screw comprising measuring the conductivity of a plug of chips leaving the screw and using the result as a control parameter for controlling downstream processing of the chips and/or fibres.
Thus, until now the back flash problem from reactor into the system has not yet been overcome effectively and the present invention seeks at least mitigating some of the problems related thereto.
Hence, an improved in-feed apparatus in which back-flash and upstream of processed material is safely avoided while continuous in-feed is achievable would be advantageous.